Final answer:
The "Companies of instruction" were set up in the early 20th century, although the exact year is not given in the reference material. This timing is inferred based on the significant medical service expansions and professionalization around this period.
Step-by-step explanation:
The "Companies of instruction" were established in the early 20th century to ensure that privates of the newly formed Hospital Corps had the necessary skills to perform their duties. Although the exact year is not specified in the provided reference material, we can infer from the historical context regarding the evolution of medical care and training within the U.S. that the establishment would likely have aligned with periods of significant medical service expansion, such as during the formation of military medical services around conflicts or during the progressive healthcare developments in the early 1900s.
While the provided references do not directly answer the question about the "Companies of instruction", they highlight the broader development of medical services and training, such as the founding of the Marine Hospital Service in 1798, the establishment of African American hospitals and training programs in the late 19th and early 20th centuries, as well as the expansion of medical services around World War I and through federal initiatives like the Hill Burton Act of 1946.
These references collectively depict a timeline wherein medical personnel were increasingly professionalized and trained, which likely paved the way for more structured training operations like the "Companies of instruction".